Numerous capacitance-type measuring devices for making linear and angular measurements have been developed wherein two support members or scales, on which are respectively mounted arrays of discrete, capacitively coupled electrodes, are displaced relative to one another, and the relative positions of the two scales are determined by sensing the resulting change in the capacitance pattern created by the arrays of electrodes. Typically, the capacitance pattern is sensed by applying a plurality of periodic signals to one of the electrode arrays and measuring the shift in signals resulting from the transfer to the other array of electrodes. Such measuring devices have a broad range of applications, from large-scale measuring devices such as three-dimensional coordinate measuring systems and numerically controlled finishing machines, to small-scale devices such as portable calipers, micrometers and the like.
In addition to the increasing popularity of the capacitance-type measuring devices, a wide variety of configurations and designs have been proposed which implement both relative and absolute measurements. While various improvements have been made to increase the capabilities of such capacitance-type measuring devices, there are still disadvantages which limit the accuracy with which measurements can be made or increase the cost of constructing the measuring devices in order to compensate for the accuracies. In particular, many applications require the ability to obtain low-cost accurate measurements even when the devices will be subjected to hostile conditions in the environments in which they are used. As a result, if the devices are extremely sensitive during use, then their use will be limited by the ability to control the application environment or the need to obtain accuracy by expensive designs or manufacturing techniques.
More specifically, in the use of capacitance-type measuring devices of the type described having two scales relatively movable with respect to one another, the gap between the scales should be uniform over the entire area of overlap. Experience and theoretical calculations show that parallelism in the x/y-plane is the most important factor affecting accuracy. This sensitivity to tilt (i.e., rotation of one scale relative to the other around an axis perpendicular to the intended measurement direction in the plane of the scales) limits the achievable accuracy at a given mechanical tolerance in the suspension system guiding the movement of the two scales relative to each other. Or, for a given accuracy specification, it may require a tolerance level on the mechanical design that is not practical or is too expensive to implement.
The present invention, therefore, has been developed to overcome the specific shortcomings in the implementation of capacitance-type measuring devices and to provide an electrode configuration which improves measurement accuracy.